The present invention relates to locking fixation assemblies used in bone fixation and more particularly to screw retention mechanisms.
Bone screws fasten spinal fixation plates and other devices to bones in the human body, or to an animal or mammal body. The fixation plates move and/or flex over time as the bones to which the screws fasten move. The screws can back out of the bone and loosen the strength with which the plate is held to the bone unless the screw is restrained from such movement and/or loosening. Mechanisms to prevent screws from backing out are described in U.S. Pat. No. 6,331,179 to Fried, or application Ser. No. 12/146,291 by Mark E. Anderson et al. But these screw retention devices require formation of a complex bearing or shaped insert that fits into that cavity and that has a complex shapes on both the interior and exterior sides of the bearing or insert. The complexity of making these complex shaped, thin walled structures is undesirable, as are the complexities of installing them. Further, these insert move during installation and use and thus make it more difficult to predict movement and stiffness of the fixation devices during use. There is thus a need for an improved way to retain bone screws.
Prior retention devices using resilient fingers used hooks on the ends of the fingers that hooked over a mating surface on the screw to prevent the screws from unscrewing. But the hooks were small and engaged only a portion of the edge of the screw, so the hooks could break during use, or during removal when the fingers and hooks were pried off the screw, generating debris and causing uncertain screw retention. There is thus a need for a screw retention mechanism that restrains screw back-out and reduces breakage and debris generation.